1. Field of the Invention
The present invention relates to an optical data storage medium, and more specifically to an optical data storage medium having a semitransparent reflective film on at least one information layer.
2. Description of the Related Art
An optical disk, exemplified by a DVD (Digital Versatile Disk) and the like, comprises two 0.6 mm thick transparent plastic substrates bonded face to face such that information layers comprised of concave and convex pits formed on the substrates constitute the bonded surfaces.
Among read-only DVD's, there is a so-called two-layer disk in which the two substrates have different informations stored in advance.
At the time of reproduction of the two-layer disk, since the reproduction is carried out by reproducing light incident on either one of the two disks, the access to the two different information layers can be advantageously made in a short period of time. The reflective layer of the first information layer on the incidence side of reproducing light is designed such that a proportion of the incident light is reflected and the rest of the incident light is transmitted therethrough. Because of this, the reproducing light can reach the second information layer present at a position distant from the incidence side of the light. The light reflected from the second information layer can be passed through the reflective layer of the first information layer and thus the reproduction of the second information layer is made possible.
The first information layer and the second information layer have a space therebetween by a transparent bonding material which is called a spacer in order not to cause interference between the respective reproducing signals. Therefore, it is possible to obtain high-quality information of reproduction from each of the two layers by setting the focal position of an object lens to the positions coincident with the respective information layers. In order to realize the above-described reproduction system, the design of the reflective film on the first information layer is very important. This reflective film, which transmits a proportion of the light, is hereinafter referred to as a semitransparent reflective film.
Heretofore, Au or Si has been generally used as a material for the semitransparent reflective film, because these substances meet the requirements for optical characteristics such as desired reflectance, desired transmittance or the like and because a thin film of these substances can be easily formed by, for example, sputtering method or the like. Additional advantages are that these substances provide a stable covering to the fine concave and,convex pits formed on the information layer and that these substances as optical storage media exhibit excellent weather resistance.
These substances, however, are associated with problems. In the case of Au, the material cost is high. Although Si is a relatively cheap material, its adhesion to a transparent adhesive and to a plastic substrate is so weak that the resistance to bending, warping, moisture absorption, and the like is not reliably sufficient. Furthermore, in comparison with metals, Si films deposited inside a sputtering chamber easily come off and produce a larger amount of particles in the sputtering process apparatus. As a result, the error rate becomes worse.
In order to solve these problems, Ag is studied as a substitute material for Au and Si.
However, since Ag is reactive to chlorine, sulfur, compounds thereof, and ions thereof, Ag is easily corroded if seawater, sweat or the like penetrates through the substrate.
In view of the above-mentioned problems, Japanese Patent Application Laid-Open (JP-A) Nos. 57-186,244, 7-3,363, and 9-156,224 disclose a technique wherein the weather resistance of Ag is improved by the addition of specific impurities thereto.
More specifically, JP-A No. 57-186,244 discloses an AgCu alloy (the content of Ag is 40 atomic % or more); JP-A No. 7-3,363 discloses an AgMg alloy (the content of Mg is equal to or more than 1 to 10 atomic %); and JP-A No. 9-156,224 discloses an AgOM (M being Sb, Pd, or Pt) alloy (the content of 0 is 10 to 40 atomic % and the content of M is 0.1 to 10 atomic %).
However, the weather resistance of these alloys, and the reflectance and transmittance of the thin film formed from these alloys are not completely elucidated. Particularly, the properties of these alloys in a thin film region for use as a semitransparent reflective film are not yet studied.